Introduction to Linux in HPC/Users and permissions

This tutorial covers the concept of users (and groups) in the multi-user operating system Linux. It will also explain the various permissions that a file or directory can have, how to determine and change them.

Video

( Slides as pdf)

Quiz

Who can change the permissions of a file?

	the owner of this file
	every user
	the super user (root)

Which command can prevent other users not in your group from writing to input.dat?

	chmod u-w input.dat
	chmod o-w input.dat
	chmod g-w input.dat

Exercises in Terminal (slide 89)

1. Create an empty file, make it read-only, make it executable.

Answer:

     Create an empty file: touch file.txt.
     make it read-only: chmod a-w file.txt; chmod a-x file.txt; chmod a+r file.txt.
     make it executable: chmod a+x file.txt.

     Explanation: The touch command can be used to create an empty file. In this example it is file.txt.
          chmod a-w file.txt; chmod a-x file.txt; chmod a+r file.txt means to execute the three commands subsequently. 
          chmod a-w file.txt removes the write permission for all users. 
          chmod a-x file.txt removes the executable permission for all users.
          chmod a+r file.txt adds the read permission for all users.
          chmod a+x file.txt makes file.txt executable for all users.

2. Start a process (e.g. sleep 10m)

Answer:

     Use a second console to look at it in top.
     Kill it.
     Hint:
        To kill the started process (e.g. sleep 10m): 
        1. Go to the terminal that started the process.
        2. Press Ctrl-C to kill it.

3. Start a process, bring it into background/foreground.

Hint:

     To bring a process to background:
        Press Ctrl-Z (pause the process).
        Type bg (bring it to background).
     To bring a process to foreground:
        Type fg (bring it to foreground).

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